1. Field of the Invention
The present invention relates to a signal detection method and an optical system, such as an optical memory, using a multi-layered object.
2. Discussion of the Background
The data density of an optical memory can be increased by decreasing a spot size of a light beam focused onto a recording medium. Since the spot size is inversely proportional to an effective numerical aperture NAEFF of the focusing optics, a larger effective numerical aperture leads to a higher recording density.
Solid Immersion Lens (SIL) technology is expected to increase the data density of an optical memory by increasing an effective numerical aperture NAEFF of an objective lens system. An optical disc apparatus using a half sphere shape of SIL is shown in a reference of K. Hirota et. al., High Density Phase Change Optical Recording Using a Solid Immersion Lens, Proc. SPIE 3401, pp.34-39 (1998). Using an SIL with an objective lens, the apparatus attains three times the effective numerical aperture, compared with a system using a conventional objective lens without SIL.
Another method to obtain a larger effective numerical aperture NAEFF uses a Solid Immersion Mirror (SIM) or a Catadioptric Optical System, such as shown in C. W. Lee et al., Technical Digest of Optical Data Storage Topical Meeting ""98, WA4-1, (1998). Also, a larger effective numerical aperture can be obtained by using a combination of lenses as an objective lens, but the effective numerical aperture NAEFF is less than 1 in this case.
In an optical microscope, the larger the effective numerical aperture of its objective lens system, the higher the resolution that can be obtained.
However, a contrast of reproduced signals detected from a multi-layered object or recording medium can be degraded to a unusable level when the objective lens system has a larger effective numerical aperture, typically larger than 0.6. In addition, when SIL or SIM is used, the contrast can sensitively depend on the distance between the recording medium and SIL or SIM, such as shown in T. D. Milster et. al., Jpn. J. Appl. Phys. Vol.38, Part 1, No. 3B, pp.1793-1794 (1999).
A proposal to improve the contrast of reproduced signals is provided by T. D. Milster et. al., Mechanism for Improving the Signal-to-Noise Ratio in Scanning Optical Microscopes, OPTICS LETTERS, Vol. 21, No. 16, pp. 1304-1306(1996). The system uses a shading band in the return path of a scanning optical microscope with effective numerical aperture NAEFF=0.5 conventional objective lens in order to increase the signal-to-noise ratio, on the basis of their analysis that shows the noise distribution is concentrated near the center of the pupil. The shading band blocks some of the low frequency signal, whereas it passes the high frequency signal.
Therefore, an object of the present invention is to provide a signal detection method and apparatus that obtain an improved signal contrast.
Another object of the present invention is to provide a signal detection method and apparatus having a stable signal contrast which is less influenced by the distance between a multi-layered object and an objective lens system having an effective numerical aperture NAEFF greater than 0.6.
These and other objects are achieved according to the present invention by producing new and improved signal detection method using a light from a multi-layer structured object, including focusing a linearly polarized light beam in the multi-layer structured object using an objective lens system having an effective numerical aperture NAEFF greater than 0.6, obtaining a modulated light beam from the multi-layer structured object, providing the modulated light beam to a filtering mask having a first area of a first transmissivity and a second area of a second transmissivity different than the first transmissivity, and detecting the modulated light beam corresponding to at least the first area of the filtering mask.
According to another aspect of the present invention, there is provided a new and improved apparatus for reproducing signals from a multi-layer structured recording medium, including a light source of a linearly polarized light beam, an objective lens system having an effective numerical aperture NAEFF greater than 0.6 and configured to focus the linearly polarized light beam onto the multi-layer structured recording medium, thereby obtaining a light beam modulated by the multi-layer structured recording medium, a filtering mask arranged to receive the modulated light beam and having a first area of a first transmissivity and a second area of a second transmissivity different than the first transmissivity, and an optical detector detecting the reflected light beam corresponding to at least the first area of the filtering mask.